1. Field of the Invention
The present invention relates to a process for preparing (meth)acrylic acid by heterogeneously catalyzed gas phase partial oxidation of at least one saturated hydrocarbon precursor compound at elevated temperature, by conducting a starting reaction gas mixture comprising the at least one saturated hydrocarbon precursor compound, molecular oxygen and at least one inert gas through at least one catalyst bed I whose catalysts I are such that their active composition is at least one multimetal oxide I which contains the elements Mo and V, at least one of the elements Te and Sb, and at least one of the elements from the group consisting of Nb, Ta, W, Ce and Ti, and whose X-ray diffractogram is an X-ray diffractogram I which has reflections h, i and k whose peak locations are at the reflection angles (2Θ) of 22.2±0.5° (h), 27.3±0.5° (i) and 28.2±0.5° (k), the reflection h being the most intense within the X-ray diffractogram I and also having a half-height width of at most 0.5°, and the half-height width of the reflection i and of the reflection k each being ≦1°.
2. Description of the Related Art
In this document, the notation (meth)acrylic acid is an abbreviation for methacrylic acid or acrylic acid.
(Meth)acrylic acid forms reactive monomers which are suitable, for example, for preparing polymers which may find use as adhesives, among other uses.
On the industrial scale, one way of preparing (meth)acrylic acid is by heterogeneously catalyzed gas phase partial oxidation of propane or isobutane.
Acrylic acid and methacrylic acid can be obtained in a mixture by heterogeneously catalyzed gas phase partial oxidation of a mixture of propane and isobutane.
Propane and isobutane are therefore referred to in this document as saturated hydrocarbon precursor compounds of (meth)acrylic acid.
Processes for preparing (meth)acrylic acid by heterogeneously catalyzed gas phase partial oxidation of at least one saturated hydrocarbon precursor compound according to the preamble of this document are known (cf., for example, EP-A 1192987, DE-A 10122027, JP-A 2000-256257, EP-A 608838, EP-A 1193240, EP-A 1238960, EP-A 962253, JP-A 10-36311, EP-A 1254706, DE-A 10051419, WO-A 99/003825, JP-A 11-57479 and DE-A 10338529).
A disadvantage of these processes is that the selectivity of the (meth)acrylic acid target compound which becomes established at a given conversion of the saturated hydrocarbon precursor compound is not fully satisfactory.
It is an object of the present invention to provide an improved process with regard to the selectivity of the target compound for a given conversion of the saturated hydrocarbon precursor compound.